Granny's right. She invariably is.

Cannabis and cigarette smoke are two different things, but cannabis smoke--and here we mean not the active ingredients but the whole smoke--is believed not to be beneficial for pulmonary health long-term. Clearly from the research that we have now, THC and CBD carry with them some amazing anti-inflammatory-anti-carcinogenic-broncho-dilating effects. But all the other stuff that burns and inhales along with those active ingredients is dirty. Fortunately, most even heavy cannabis smokers don't take it in at the same rate they do cigarettes. Even ingested in light amounts, you're not going to find anyone with any medical sense who'll encourage others to inhale burning vegetable matter into their lungs at a high--or even a low or moderate--rate.